Electronic inks and displays and image displaying methods

ABSTRACT

An electronic ink containing charged particles includes a combination of resin particles, a pigment, and a charge director. The resin particles exhibit an average particle size less than 1.0 micron and contain a resin that exhibits a molecular weight of 500 to 20,000. The pigment is loaded on the resin particles. The charge director can physically associate with the resin particles. The charged particles may be negative or positive.

BACKGROUND

Among the wide variety of known electronic displays, some involveelectronically controlling the location of charged particles suspendedin a fluid. Electrophoretic displays represent one type of electronicdisplay and involve moving the charged particles suspended in the fluidwith a Coulombic force exerted on the particles by an applied electricalsignal. Some electronic displays are referred to as electronic paper ore-paper, since they can be thin and flexible with paper-like imagequality. Electronic displays may use transmitted light, but some useonly reflected light.

While a variety of technological approaches have been attempted,opportunities for improvement abound. For example, a challenge exists inproducing a bright, full-color image in an electronic display using onlyreflected light. Unique conditions exist under which light is reflectedand charged particles are moved about in a pixel of the display. As aresult, technology borrowed from known electrophoretic fluids, such asliquid electrophoretic toner (LEP toner) used in offset printing, hasnot performed adequately in the electronic display application.

Known electrophoretic fluids may rely on providing a pigment capable ofadsorbing a charge or may rely on a combination of a pigmentencapsulated by a polymer to provide the charged particles. However,encapsulation is often done in situ during polymerization, where thepigment chemistry and polymer chemistry are interdependent, such thatsome polymers are only compatible with certain pigments. Also, pigmentchemistry may be influential of particle charge, yielding particlecharges that may vary between colors. Such incompatibilities and otherproblems give rise to a search for improved suspensions of chargedcolorant particles for which particle location can be electronicallycontrolled. These suspensions may be referred to as electronic inks.Some electronic inks may be referred to as electrophoretic inks wherethe charged particles may be moved with a Coulombic force exerted on theparticles by an applied electrical signal.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In an embodiment, an electronic ink containing charged particlesincludes a combination of resin particles, a pigment, and a chargedirector. The resin particles exhibit an average particle size less than1.0 micron and contain a resin that exhibits a molecular weight of 500to 20,000.

Understandably, the pigment, charge director, and/or other componentsadded to the resin particle may provide a larger ink particle, but theembodiments herein provide an ink with an average ink particle size alsoless than 1.0 micron. Although minima and/or maxima are listed for theabove described ranges and other ranges designated herein, more narrowincluded ranges may also be desirable and may be distinguishable fromprior art. Also, the Examples herein may provide a basis for such morenarrow included ranges. Throughout the present document, indication of amolecular weight refers to weight average molecular weight. Unlessotherwise described, listing a range of molecular weights for a resinindicates that individual polymer molecules of the resin exhibitmolecular weights that are distributed within the range.

The resin may exhibit a molecular weight of 1,000 to 5,000. The resinmay be a thermoplastic resin exhibiting a melting point of greater than50° C., including greater than 90° C. Notably, a low melting point ofthe resin may limit operating temperature of a device using theelectronic ink to avoid degrading the ink. Even so, high melting pointresins may exhibit too high of a molecular weight. As one example, theresin may be a wax resin. The pigment is loaded on the resin particles.The charge director can physically associate with the resin particles.

Providing ink particles less than 1.0 micron in size may facilitateimaging in displays with pixels having a dimension of 100 microns orless, and may reduce optical scattering for displays based onsubtractive color, which use absorption dominated behavior. The pixeldimension may be the length or width with respect to the viewing plane.The small particle sizes described herein may become increasinglysignificant with smaller pixel dimensions, for example, 5 to 50 microns,and even 10 to 25 microns.

By way of example, the charge director may form a micelle structurephysically associated, but not chemically associated, by hydrophobicbonding with the resin particles to provide at least part of theparticle charge. Hydrophobic bonding or, more appropriately, hydrophobicinteraction represents a well-known phenomenon that occurs in micellularstructures. Essentially, in nonpolar solvents, hydrophilic heads ofamphiphilic molecules orient the molecules so as to assemble thehydrophilic heads together inside the micelle with hydrophobic tailsassembled outside at the micelle surface. Hydrophobic bonding is alsowell-known not to infer chemical bonding, but rather a repulsivephysical interaction between hydrophobic portions of molecules and anonpolarized material, such as the resin surface.

Depending partly on the resin selected, the charged particles of theelectronic ink may be negative or positive. Speaking generally, anelectronic ink containing negatively charged particles may use an acidicresin while an electronic ink containing positively charged particlesmay use a basic resin. For the negative ink, the resin may include acopolymer of polyethylene grafted with maleic anhydride or apolyethylene-based ionomer. The negative ink resin particles may consistof the copolymer or the ionomer. The ionomer may bepoly(ethylene-co-acrylic acid) zinc salt. The acidic resin may exhibit amolecular weight of 1,000 to 5,000, including 1,000 to 3,000. For thepositive ink, the resin may include vinyl pyrrolidone/triacontenecopolymers. Additional possible basic resins include polyamines,polyamides, and potentially others. The positive ink resin particles mayconsist of vinyl pyrrolidone/triacontene copolymer. The basic resin mayexhibit a molecular weight of 1,000 to 5,000, including 3,000 to 4,500.

Correspondingly, the charge director may be basic for the negative inkand may be acidic for the positive ink. One example of a charge directorfor negative ink includes sulfosuccinic acid, ditridecyl ester metalsalt. The metal salt may be a barium salt. The charge director mayconsist of the metal salt. One example of a charge director for positiveink includes polyisobutylene succinimide polyamines. The charge directormay consist of the polyisobutylene succinimide polyamines. Other knowncharge directors may be used, for example, those described in WIPOPublication No. WO/2007/130069 (Application No. PCT/US2006/018297)entitled “Charge Director for Liquid Toner” may be suitable.

For either positive or negative ink, the resin particles may exhibit amaximum particle size less than 2.0 microns, perhaps even less than 1.0micron. Another common characteristic among the resins involves theproperty of being compatible with a cyan pigment, a magenta pigment, ayellow pigment, a black pigment, and combinations thereof. Suchcompatibility allows development of CMYK color systems derivable fromthe same resin/charge director composition. Potentially, a pixel of anelectronic display could contain one set of particles with one of theCMYK pigments loaded thereon as well as another set of particles with adifferent one of the CMYK pigments loaded thereon. Since both sets ofparticles may be derived from the same resin/charge directorcomposition, their charges and performance may be equivalent and ariseindependent of pigment chemistry. Incompatibilities are thus less likelyto exist.

Also, such compatibility allows tuning or adjustment of the color gamutsince the resin exhibits compatibility with combinations of pigments.Individual particles may include more than one of the CMYK pigmentsand/or other base or secondary pigments and may exhibit any color fromvarious pigment combinations, such as, any color within the availablePantone spot color space. For example, a monochrome display with onlyone color of particle is not limited to the color of a single pigmentbut may be any color capable of being derived from a mixture of one ormore different pigments.

As described in Examples 2-4, pigment loading may also be varied toprovide for adjustment in color depth and even to achieve high colordepth in a pixel of an electronic display by optimizing optical density.Pigment loading may be 1-99 weight percent (wt %) of the solids,including 5-95 wt %, or even 5-85 wt %. By providing a resin/chargedirector composition that functions independent of pigment chemistry,significant flexibility exists in the applications for electronicdisplays using the described electronic inks.

The combination of resin particles and pigment, wherein the particlescontain a resin, may be contrasted with particles produced from in situencapsulation during polymerization or from other similar knowntechniques. In the combination, the starting materials include a solid(resin particles) and pigment, and processing yields particles of thesolid resin loaded with pigment. Known in situ particles result frompolymerization of precursor chemicals in solution in conjunction withencapsulation of pigment also in solution. No combination of resinparticles and pigment occurs during known in situ encapsulation since noresin particles exist in the precursor solution. Instead, the onlycombining that occurs involves polymerization precursors and pigment.

The known combination of polymerization precursors and pigment cannot beconsidered to constitute or produce a combination of resin particles andpigment. It merely produces polymer-encapsulated pigment, as compared toresin particles loaded with pigment. As appreciated from the discussionherein, identifiable differences exist between resin particles loadedwith pigment and known polymer-encapsulated pigment, not the least ofwhich include enabling the use of various combinations of pigments witha given resin/charge director composition.

The electronic ink containing negatively charged particles may include acombination of resin particles, a pigment, a charge director and acharge adjuvant. The charge adjuvant may chemically bond to the resinparticles. Examples of a charge adjuvant include metallic soapscontaining a metal, such as Al, Zn, Ca, Mg, other metals, andcombinations thereof, and a ligand, such as stearate, oleate, otherligands, and combinations thereof. Two examples include aluminumtristearate and aluminum distearate. Other known charge adjuvants may beused, for example, those described in WIPO Publication No.WO/2008/085709 (Application No. PCT/US2007/088627) entitled “ChargeAdjuvants in Electrostatic Inks” may be suitable. Charge adjuvants mightbe used that physically associate, but do not chemically bond, with theresin, for example, white pigment (TiO₂) solids impregnated in negativeresin or TPP (triphenyl phosphine) solids impregnated in a positiveresin.

Essentially, the charge adjuvant provides a molecular structure to trapcharge director molecules around a resin particle. Hence, as shown inExample 5 below, particle conductivity may increase. Without beinglimited to any particular theory, it is believed that an equilibriumexists between free charge and the charge director physically associatedwith particles and the equilibrium is exhibited in the particleconductivity. When aluminum tristearate was used, it is hypothesizedthat the equilibrium shifted to a lower volume of free charge, increasedcharge director association with the particles, and enhanced particleconductivity. Particle conductivity may be greater than 50 picosiemens(pS), for example, greater than 200 pS.

For the copolymer of polyethylene grafted with maleic anhydride, sincehydrolysis of the maleic anhydride provides two acid sites, it isbelieved that both acid sites bind to the aluminum atom of aluminumdistearate, releasing both stearic acid molecules and leaving nomolecular structure to trap charge director molecules. However, use ofaluminum tristearate allows both acid sites to react with the aluminumatom, still releasing two stearic acid molecules, but keeping onestearic acid molecule bound to the aluminum to provide a molecularstructure for trapping charge director molecules.

The charge adjuvant may also provide a dispersing agent. For example,the charge adjuvant may include a metallic soap and the resin mayprovide an acidic surface that reacts with the charge adjuvant andreleases the dispersing agent from the charge adjuvant. From thediscussion above regarding trapping the charge director, it is notedthat aluminum distearate and aluminum tristearate are metallic soaps.Also, the copolymer of polyethylene grafted with maleic anhydride andpoly(ethylene-co-acrylic acid) zinc salt may provide a resin with anacidic surface. Further, stearic acid may function as a dispersing agentin an electronic ink. Consequently, the release of stearic acid from thecharge adjuvant reaction with the resin constitutes release of adispersing agent. It will be appreciated that other metallic soaps orfatty acid salts may be used in combination with other acidic resins toachieve a similar result. As may be understood from the discussion aboveregarding charge adjuvants and charge directors, selection of componentsmay influence dispersion stability and chargeability of particles.

In addition to a charge adjuvant providing a dispersing agent, thecharge adjuvant may function as a viscosity control agent in a methodfor making electronic ink. In one embodiment, a method for makingelectronic ink includes providing a resin exhibiting a molecular weightof 500 to 20,000 and a pigment. While processing the resin and pigmenttogether, the method includes forming resin particles containing theresin and loading the pigment on and dispersing the pigment among theresin particles. The pigment-loaded resin particles exhibit an averageparticle size less than 1.0 micron. The method includes charging thepigment loaded resin particles by physically associating a chargedirector.

Examples 1-11 below describe formation and property evaluation ofvarious electrophoretic inks using such a method. In the Examples, agrinding mill or ball mill is used to reduce the size of resin particlesand to disperse and load pigment on the resin particles, but otherparticle size reduction apparatuses may be used. In general, grindingresin particles and dispersing pigment is widely practiced in producingtoner particles for printing. Methods such as those described in U.S.Pat. No. 6,623,902 entitled “Liquid Toner and Method of Printing UsingSame” and other known methods may be adapted to the embodiments herein.However, resins having a molecular weight of 500 to 20,000, andespecially wax resins having a molecular weight of 1,000 to 5,000, arenot used in producing toner particles. Instead, printing on paperinvolves use of resins with increased toughness and durability to affordthe flaking, peeling, rub resistance, etc. fixing parameters importantwhen printing on paper.

Resins discussed in the present document are instead used to movepigment within an electronic pixel. Despite the differences, the presentdocument provides adequate details enabling those of ordinary skill toadapt known toner resin grinding and pigment dispersion techniques toproducing electronic ink.

A viscosity control agent assists in maintaining viscosity of startingmaterials combined in a resin grinding and pigment dispersion process toadequately reduce particle size. During the processing, depending onphysical properties of the resin and pigment and the operatingconditions for grinding, pigment may become encapsulated by resin whenloading it on the resin, though encapsulation is not required. Aviscosity control agent may be selected that, after grinding, functionsas a charge adjuvant. As discussed above, the charge adjuvant may alsorelease a dispersing agent. Understandably, adjusting the level ofviscosity control agent may affect the ultimate particle conductivity.

Using a low molecular weight resin has been discovered to yield averageparticle sizes less than 1.0 micron. Such small particle sizes were notpreviously obtained even in similar methods used to make tonerparticles. For example, NUCREL 699 (a copolymer of ethylene andmethacrylic acid available from El du Pont de Nemours in Wilmington,Del.) commonly used in toner was not grindable to sub-micron particles.It is hypothesized that, since toner particles involve higher molecularweight resins to afford proper fixing parameters, the resins do not giveway to small particle sizes.

As indicated, producing negatively charged particles in electronic inkmay include providing acidic resin. However, observation indicates thatacid groups in resin may invoke hydrogen bond cross-linking, makingparticle size reduction less effective. In the presence of acid groupcross-linking via hydrogen bonds, difficulty may be encountered inproducing resin particles exhibiting an average particle size less than1.0 micron. However, identification of suitable techniques forovercoming hydrogen bonding in addition to those described herein isconceivable.

Notably, poly(ethylene-co-acrylic acid) zinc salt and copolymers ofpolyethylene grafted with maleic anhydride include “blocked” acidgroups. In the copolymer including maleic anhydride, maleic acid groupsare blocked by the existence of the anhydride. Maleic acid groups may beunblocked by hydrolysis, providing an acidic surface. Hydrolysis may beaccomplished with the addition of water, for example, during anappropriate phase in particle size reduction, such as before addingcharge adjuvant. In the copolymer including acrylic acid salt, acrylicacid groups are blocked by reaction with a metal base, specifically azinc base, to produce a metal salt. Acrylic acid groups may be unblockedby dissociation of the metal ion. Dissociation may be accomplished withthe addition of solvent, for example, during an appropriate phase inparticle size reduction or thereafter, when adding carrier fluid. Otherblocking/unblocking schemes are conceivable relying on the same types ofchemical bonds or other types of chemical bonds, such as partialesterification of acid groups. Also, it is conceivable that at leastsome of the acid groups of a resin may be blocked, with the possibilitythat other acid groups are not blocked. Even so, all of the acid groupsof a resin might be blocked.

The presence of ionic acid groups bound to a metal ion in the saltincreases polarity of the resin and enhances charging. In addition, aviscosity control agent and/or a charge adjuvant may react with acidgroups to provide the benefits described herein. By using resins withblocked acid groups, particle size reduction may proceed with lesshindrance while still providing acid groups for producing negativelycharged particles.

While a charge adjuvant may also provide a dispersing agent, asdiscussed above, a dispersing agent may be provided in addition to orinstead of a dispersing agent provided by a charge adjuvant. In thecontext of LEP toner, a dispersing agent is of little significance.However, for electronic ink, high particle mobility may enhance imagingin an electronic display. Often, electronic displays involve eithercompacting or dispersing charged particles across the pixel using anelectrical signal. Since many signal application cycles may be appliedto repeatedly compact and disperse charged particles, an effectivedispersing agent, whether added or provided by the charge adjuvant, maybe helpful.

Observation has indicated that particle size also contributes toparticle mobility. That is, achieving a small particle size andproviding a dispersing agent both contributed to high mobility particletransport. For less than 1.0 micron particles in a pixel with electrodesseparated 10 to 30 microns, a particle conductivity of greater than 200pS may provide a visible change in particle compaction/dispersion inless than 1 second.

In one embodiment, an electronic display includes a pixel, an electrodein the pixel, and electronic ink in the pixel. As described elsewhereherein, the ink contains charged particles that include a combination ofresin particles, a pigment, and a charge director. Average particle sizemay be less than 1.0 micron. A dispersing agent may be provided,enhancing particle mobility. Various types and configurations ofelectrodes known to those of ordinary skill may be used, including bareelectrodes contacting the ink and/or electrodes coated so as not tocontact the ink.

Negatively charged particles may contain a resin copolymer ofpolyethylene grafted with maleic anhydride or a resin polyethylene-basedionomer. The combination may further include a charge adjuvant thatchemically bonds to the resin particles. Positively charged particlesmay contain a resin vinyl pyrrolidone/triacontene copolymer.

In another embodiment, an image displaying method includes providing anelectronic display including a pixel allowing visible light to enter andexit the pixel, an electrode in the pixel, and electronic ink in thepixel. As described elsewhere herein, the ink contains charged particlesthat include a combination of resin particles, a pigment, a chargedirector, and a dispersing agent. The method includes applying anelectrical signal to the pixel using the electrode and compacting thecharged particles using the electrical signal. The electrical signal ischanged and the charged particles are dispersed across the pixel. Thepigment is loaded on the resin particles, the resin exhibiting theproperty of being compatible with a cyan pigment, a magenta pigment, ayellow pigment, a black pigment, other pigments, and combinationsthereof. The charge director physically associates with the resinparticles.

By way of example, the method may include repeatedly compacting anddispersing during at least 10 signal application cycles withoutsubstantial degradation of the charged particles. Practically, thecycling may occur millions of times in an electronic display. However,even a few cycles readily distinguish attempted use of liquidelectrophoretic (LEP) toner as an electronic ink. LEP toner was observedto cycle only once or twice as a result of particle degradation. It isconceivable that the method may operate for resin particles as large as2.0 microns, however, based partly on the expected small dimensions ofelectronic pixels and the desire to reduce optical scattering,performance advantages exist for resin particles less than 1.0 micron.

Negatively charged particles may include a resin copolymer ofpolyethylene grafted with maleic anhydride or a resin polyethylene-basedionomer, both resins exhibiting a molecular weight of 1,000 to 3,000.Positively charged particles may include a resin vinylpyrrolidone/triacontene copolymer exhibiting a molecular weight of 3,000to 4,500.

The Examples below describe various additional embodiments.

EXAMPLE 1

A-C 575 wax resin (copolymer of polyethylene grafted with maleicanhydride provided as a powder available from Honeywell in Morristown,New Jersey) was used exhibiting a molecular weight of 1,000 to 3,000, amelting point of 106° C. by Mettler drop technique (ASTM D-3954), and asaponification number of 34 mg KOH/g. The A-C 575 was put in a S-0ATTRITOR batch grinding mill available from Union Process Co. (Akron,Ohio) along with blue 15:3 cyan pigment (available from Toyo Ink Mfg.Co., Ltd in Tokyo, Japan), aluminum distearate viscosity control agent(VCA), and ISOPAR L liquid carrier (isoparaffinic solvent available fromExxon Mobile Corp. in Fairfax, Va.). The formulation in the millcontained 78 parts wax resin, 14 parts pigment, and 8 parts VCA on asolids weight basis in enough ISOPAR L to provide 18 weight percent (wt%) nonvolatile solids (NVS) during grinding. Pigment loading was 14 wt%.

After grinding for at least 6 hours at 35° C., the resulting dispersionexhibited a particle size distribution with an average of 0.385micrometers (microns) and a maximum of 0.632 microns as determined usinga MASTERSIZER 2000 particle analyzer available from Malvern InstrumentsLtd. in Worcestershire, UK. A scanning electron microscope (SEM) photoshowed a block structure for the particles and an average particle sizeof 0.8 microns. Viscosity at 8.4% NVS was 411 centipoise (cP).

The dispersion was negatively charged with 50 mg of sulfosuccinic acid,ditridecyl ester barium salt charge director per gram of NVS. Thecharged dispersion was diluted to 2 wt % NVS with ISOPAR L and, in a 1millimeter deep charge-to-mass ratio (Q/m) test cell, exhibited lowfield conductivity of 88 picosiemens/centimeter (pS/cm) and high fieldconductivity of 325 pS/cm. Using a film of electroplated particles fromthe charged dispersion, optical density was 1.56 at a defined mass perarea (DMA) of 0.084milligram/centimeter² (mg/cm²). The 2% NVS chargeddispersion was put in an electronic chamber having dimensions of100×100×10 microns deep with two interdigitated electrodes 30 micronsapart. The colored, charged ink particles were observed moving betweenthe electrodes under an alternated voltage.

EXAMPLE 2

The method of Example 1 was followed except the formulation in the millcontained 47 parts wax resin, 45 parts pigment, and 8 parts VCA on asolids weight basis. Also, aluminum tristearate was used instead ofaluminum distearate VCA. Pigment loading was 45 wt %.

The dispersion was negatively charged and diluted as in Example 1. The2% NVS charged dispersion was put in an electronic chamber as in Example1 and the colored, charged ink particles were observed moving betweenthe electrodes under an alternated voltage. Even though the particleshad 45% pigment loading instead of the 14% pigment loading of Example 1,it was observed that the extent of dispersion of the charged particlesstill affected color depth.

EXAMPLE 3

The method of Example 2 was followed in forming a dispersion except theformulation in the mill contained 29 parts wax resin, 63 parts pigment,and 8 parts VCA on a solids weight basis. Pigment loading was 63 wt %.The dispersion was negatively charged and diluted as in Example 1.

EXAMPLE 4

The 2% NVS charged dispersions of Examples 1, 2, and 3 were each dilutedby 100 to a 0.02% dispersion to correlate optical density results fromthe 1 mm deep Q/m test cell to a 10 micron (0.01 mm) deepelectrophoretic cell of a display device. The diluted 0.02% NVS chargeddispersions of the Examples 1, 2, and 3 dispersions respectivelyexhibited optical densities of 0.42, 0.9, and 1 at a DMA of 0.013mg/cm². Data analysis yielded a linear relationship between pigmentloading and optical density allow extrapolation to an optical density ofabout 1.2 for 100% pigment loading.

EXAMPLE 5

The method of Example 2 was followed in forming a dispersion exceptaluminum distearate was used instead of aluminum tristearate VCA. Thedispersion was negatively charged and diluted as in Example 1. The 2%NVS charged dispersion was put in an electronic chamber as in Example 1and the colored, charged ink particles were observed moving between theelectrodes under an alternated voltage.

A study of the effect of low field (LF) conductivity on particleconductivity was undertaken in the Q/m test cell comparing performanceof the Example 2 aluminum tristearate VCA to the Example 5 aluminumdistearate VCA. A sharp increase in particle conductivity at 45 pS LFwas observed for the dispersion using aluminum distearate, reaching 200pS at low field conductivity levels above about 80 pS. But a similarlysharp increase was observed at a lower level of 25 pS LF for thedispersion using aluminum tristearate, reaching 200 pS at low fieldconductivity levels above about 65 pS. Consequently, particleconductivity was higher at all LF levels above 25 pS for the aluminumtristearate.

EXAMPLE 6

The Example 1 A-C 575 wax resin was put in a S-0 ATTRITOR batch grindingmill along with 0.2 g of water, which constituted 2 equivalents based onthe saponification number to fully hydrolyze the maleic anhydride todi-acid, and was ground for 2 hours in enough ISOPAR L to provide 18 wt% NVS during grinding. Thereafter, 2 mg of sulfosuccinic acid,ditridecyl ester barium salt charge director per gram of wax resin wasadded to encapsulate excess water, if any, in micelles to preventsubsequent hydrolysis of VCA and was ground for 2 hours. Blue 15:3(TOYO) cyan pigment and aluminum tristearate VCA were added with enoughISOPAR L to provide 18 wt % NVS and ground for 6 hours. The finalformulation in the mill contained 47 parts wax resin, 45 parts pigment,and 8 parts VCA on a solids weight basis. Pigment loading was 45 wt %.All grinding was at 35° C. and yielded a particle size of 0.8 microns inthe dispersion.

EXAMPLE 7

Several samples of the dispersion of Example 6 and the dispersion ofExample 2 were negatively charged with amounts of sulfosuccinic acid,ditridecyl ester barium salt charge director varying from 5 to 50 mg pergram of NVS. The charged dispersions were diluted to 2 wt % NVS withISOPAR L and evaluated in the Q/m test cell. Over the range of chargedirector amount, particle conductivity was higher for the Example 6charged dispersion including resin pretreated with water. For 20 to 50mg/g of charge director, particle conductivity was noticeably higher byabout 70 to 90 pS. For the Example 2 charged dispersion, particleconductivity exceeded 200 pS for charge director amounts above about 40mg/g. For the Example 6 charged dispersion, particle conductivityexceeded 200 pS for charge director amounts above about 30 mg/g. It ishypothesized that the water pretreatment allowed increased chargedirector association with the particles.

EXAMPLE 8

ACLYN 295 wax resin (ethylene-acrylic acid zinc ionomer provided asgranules available from Honeywell in Morristown, N.J.) was usedexhibiting a molecular weight of 1,000 to 3,000, a melting point of 99°C. by differential scanning calorimetry, and an acid number of nil(since it is an acid salt). The ACLYN 295 was put in a S-0 ATTRITORbatch grinding mill along with blue 15:3 (TOYO) cyan pigment, aluminumdistearate VCA, and ISOPAR L liquid carrier. The formulation in the millcontained 47 parts wax resin, 45 parts pigment, and 8 parts VCA on asolids weight basis in enough ISOPAR L to provide 18 wt % NVS duringgrinding. Pigment loading was 45 wt %. After grinding for at least 6hours at 35° C., the resulting dispersion exhibited an average particlesize of 0.8 microns as determined using a MASTERSIZER 2000 particleanalyzer. An SEM photo of the particles showed an average particle sizeof 1.2 microns. Pigment particles with a size of about 70 to 100nanometers (nm) were dispersed and loaded on the wax resin particles.

The dispersion was negatively charged with sulfosuccinic acid,ditridecyl ester barium salt charge director. The charged dispersion wasdiluted to 2 wt % NVS with ISOPAR L and exhibited particleconductivities in the Q/m test cell, depending on level of low fieldconductivity, of about 2 to 8 times that of the Example 2 chargeddispersion using equivalent amounts of charge director. The Example 8charged dispersion exceeded 200 pS at levels of low field conductivityabove about 25 pS and reached over 450 pS at low field conductivity ofabout 50 pS. The Example 2 charged dispersion exceeded 200 pS at levelsof low field conductivity above about 65 pS and approached 450 pS at lowfield conductivity of about 110 pS. The 2% NVS charged dispersion wasput in an electronic chamber having dimensions of 100×100×10 micronsdeep with two interdigitated electrodes 30 microns apart. The colored,charged ink particles were observed moving between the electrodes underan alternated voltage.

EXAMPLE 9

Additional dispersions were prepared following the method of Example 8but using less VCA in comparison to the 8 wt % VCA dispersion in Example8. A 4 wt % VCA dispersion included 51 parts wax resin, 45 partspigment, and 4 parts VCA on a solids weight basis. A 0 wt % VCAdispersion included 55 parts wax resin, 45 parts pigment, and 0 partsVCA on a solids weight basis. Pigment loading in all dispersions was 45wt %. Each dispersion exhibited an average particle size of 0.8 micronsas determined using a MASTERSIZER 2000 particle analyzer.

Several samples of the 8, 4, and 0 wt % dispersions were negativelycharged with amounts of sulfosuccinic acid, ditridecyl ester barium saltcharge director varying from 5 to 50 mg per gram of N. The chargeddispersions were diluted to 2 wt % NVS with ISOPAR L and evaluated inthe Q/m test cell. Over the range of charge director amount, particleconductivity was higher for 8 wt % than for 4 wt % and particleconductivity was higher for 4 wt % than for 0 wt %. Differences inparticle conductivity were most noticeable in the range of 20 to 50 mg/gof charge director.

Using the samples with 50 mg/g charge director, a study of the effect ofLF conductivity on particle conductivity was undertaken in the Q/m testcell comparing performance of the 8, 4, and 0 wt % VCA dispersions.Between 11 and 51 pS LF conductivity, the particle conductivityexhibited by the 8 and 4 wt % dispersions was about the same, but bothwere greater than the 0 wt % dispersion.

EXAMPLE 10

The method of Example 8 was followed except 1 wt % IRCOSPERSE 2155(aliphatic succinimide dispersant available from Lubrizol, Ltd. inManchester, UK) was added to the 2% NVS charged dispersion. A minoreffect on particle conductivity was observed with the addition of thedispersant. The 2% NVS charged dispersion was put in an electronicchamber having dimensions of 100×100×10 microns deep with twointerdigitated electrodes 30 microns apart. The colored, charged inkparticles were observed moving between the electrodes under analternated voltage. In comparison to the Example 8 dispersion, theExample 10 dispersion cycled more quickly multiple times between statesof particles dispersed and particles compacted at the electrodes.

EXAMPLE 11

ANTARON WP-660 wax resin (vinyl pyrrolidone/triacontene copolymerprovided as flakes available from International Specialty Products inWayne, New Jersey) was used exhibiting a molecular weight of 3,000 to4,500 and a melting point of 58-68° C. The WP-660 was put in a S-0ATTRITOR batch grinding mill along with Permanent Carmine FBB02 magentapigment (available from Clariant Intl. Ltd. in Switzerland) and ISOPARL. The formulation in the mill contained 55 parts wax resin and 45 partspigment on a solids weight basis in enough ISOPAR L to provide 18 wt %NVS during grinding. Pigment loading was 45 wt %. After grinding for atleast 6 hours at 35° C., the resulting dispersion exhibited a particlesize distribution with an average of 0.7 microns and a maximum of 1.2microns as determined using a MASTERSIZER 2000 particle analyzer.

A first sample of the dispersion was positively charged with enough OLOA1200 (polyisobutylene succinimide polyamines available from ChevronOronite in San Francisco, California) to yield initial low fieldconductivity of 80 pS in the Q/m test cell and was allowed to standovernight. Subsequent Q/m test cell readings showed low fieldconductivity of 61 pS and high field conductivity of 96 pS. Some of theink plated on the Q/m test cell negative electrode, evidencing existenceof a positive ink. Conductivity readings were considered too low,consequently, charge director load in a second sample of the dispersionwas increased sufficiently to yield initial low field conductivity of200 pS (about 1 wt % charge director) and to give the appearance thatall particles were charged positively. The 2% NVS charged dispersion ofthe first and second samples were put in an electronic chamber havingdimensions of 100×100×10 microns deep with two interdigitated electrodes30 microns apart. The colored, charged ink particles were observedmoving between the electrodes under an alternated voltage.

1. An electronic ink comprising positively charged particles thatinclude a combination of: resin particles exhibiting an average particlesize less than 1.0 micron and containing a resin that exhibits amolecular weight of 500 to 20,000; a pigment loaded on the resinparticles; and a charge director that physically associates with theresin particles.
 2. The ink of claim 1 wherein the resin exhibits a MWof 1,000 to 5,000.
 3. The ink of claim 1 wherein the resin comprises avinyl pyrrolidone/triacontene copolymer.
 4. The ink of claim 1 whereinthe resin exhibits the property of being compatible with a cyan pigment,a magenta pigment, a yellow pigment, a black pigment, other pigments,and combinations thereof.
 5. The ink of claim 1 further comprising adispersing agent.
 6. The ink of claim 1 wherein the charge directorforms a micelle structure physically associated by hydrophobic bondingwith the resin particles to provide at least part of the particlecharge.
 7. The ink of claim 1 wherein the charge director comprisespolyisobutylene succinimide polyamines.
 8. An electronic displaycomprising: a pixel; an electrode in the pixel; and electronic ink inthe pixel, the ink containing positively charged particles that includea combination of: resin particles exhibiting an average particle sizeless than 1.0 micron and containing a resin vinylpyrrolidone/triacontene copolymer; a pigment loaded on the resinparticles; and a charge director that physically associates with theresin particles.
 9. The display of claim 8 wherein the resin exhibits aMW of 500 to 20,000.
 10. The display of claim 8 wherein the resinexhibits a MW of 1,000 to 5,000.
 11. The display of claim 8 wherein theresin particles consist of vinyl pyrrolidone/triacontene copolymer. 12.The display of claim 8 wherein the resin exhibits the property of beingcompatible with a cyan pigment, a magenta pigment, a yellow pigment, ablack pigment, other pigments, and combinations thereof.
 13. The displayof claim 8 wherein the charge director forms a micelle structurephysically associated by hydrophobic bonding with the resin particles toprovide the charged particles.
 14. The display of claim 8 wherein thecharge director comprises polyisobutylene succinimide polyamines.
 15. Animage displaying method comprising: providing an electronic displayincluding a pixel allowing visible light to enter and exit the pixel, anelectrode in the pixel, and electronic ink in the pixel, the inkcontaining positively charged particles that include a combination of:resin particles containing a resin vinyl pyrrolidone/triacontenecopolymer exhibiting a molecular weight of 500 to 20,000; a pigmentloaded on the resin particles, the resin exhibiting the property ofbeing compatible with a cyan pigment, a magenta pigment, a yellowpigment, a black pigment, other pigments, and combinations thereof; acharge director that physically associates with the resin particles; anda dispersing agent; applying an electrical signal to the pixel using theelectrode and compacting the charged particles using the electricalsignal; and changing the electrical signal and dispersing the chargedparticles across the pixel.
 16. The method of claim 15 wherein the resinparticles exhibit an average particle size less than 1.0 micron.
 17. Themethod of claim 15 wherein the resin particles consist of vinylpyrrolidone/triacontene copolymer.
 18. The method of claim 15 whereinthe charge director forms a micelle structure physically associated byhydrophobic bonding with the resin particles to provide the chargedparticles.
 19. The method of claim 15 wherein the charge directorcomprises polyisobutylene succinimide polyamines.
 20. The method ofclaim 15 further comprising repeatedly compacting and dispersing duringat least 10 signal application cycles without substantial degradation ofthe charged particles.